Two types of secondary batteries have been developed within the last ten years, containing NiOOH electrodes as the positive electrodes and hydrogen electrodes as the negative electrodes. The first system (cf. eg. Swiss Pat. No. 495,060, German Offenlegungsschrift No. 2,160,202, German Offenlegungsschrift No. 2,200,806) is characterized by the fact that the negative electrode is designed as a hydrogen storage electrode; these cells do not require a pressure housing and usually operate with an alkaline electrolyte moving freely about the cell.
Nickel oxide-hydrogen cells are also known (German Auslegeschrift No. 2,111,172, U.S. Pat. No. 3,867,199) in which hydrogen is stored under pressure in the cell housing or in metal hydrides inside or outside the cell housing. Cells according to this second system therefore require a pressure-resistant housing which is impermeable to H.sub.2 ; the negative electrode acts as a catalyzing electrode, as is known in fuel cells. Under all operating conditions, it is important for its function that the alkaline electrolyte be in the separator and in the electrodes. The present invention relates to this second type of nickel oxide-hydrogen cell.
Since the original work done by Tsenter et al. (German Auslegeschrift No. 2,111,172, U.S. Pat. No. 3,669,744) and Dunlop et al. (U.S. Pat. No. 3,867,199) in 1971 and 1972, these nickel oxide-hydrogen cells have been of particular interest for special applications. Their advantages include good reliability and freedom from maintenance, long life-time, good power rating, and an energy content of 40-70 Wh/Kg; their disadvantages include the high cost, due to the plurality of expensive hydrogen electrodes catalyzed with noble metals, the equally costly positive electrodes, and the costly (by comparison with other known batteries) pressure housing. For this reason, such nickel oxide-hydrogen cells have thus far only been used to a very limited extent despite their good operating characteristics.
For an improved understanding of the function of these cells, the reaction will be described at this point, and is represented by the following equations:
__________________________________________________________________________ NiOOH + H.sub.2 O +e.sup.- ##STR1## Ni(OH).sub.2 + OH.sup.- (positive electrode) 1/2H.sub.2 + OH ##STR2## H.sub.2 O + e.sup.- (negative electrode) NiOOH + 1/2H.sub.2 ##STR3## Ni(OH).sub.2 (Total reaction) With overcharging: ##STR4## (positive electrode) ##STR5## (negative electrode) ##STR6## (Total reaction) recombine on negative electrode With polarity reversed: ##STR7## (positive electrode) ##STR8## (negative electrode) __________________________________________________________________________
It is apparent from the reaction equations that the hydrogen pressure in the cell is proportional to the state of charge and is therefore readily measured, another advantage of the system. It is also clear that when the cell is overcharged or the polarity is reversed, no change occurs in the hydrogen pressure, except for that produced by temperature changes in the cell. If a battery is assembled with discharged positives and a low hydrogen precharge is provided, the system will be largely resistant to polarity reversal until the positives are damaged.
Previous embodiments of this type of cell have been disclosed as the state of the art, based primarily upon the previously cited fundamental patents:
1. The cell described by Tsenter et al. is characterized by high operating pressure, which can reach approximately 60-200 bars, and also a thickness ratio of the negative to the positive electrode of 1:1 to 1:20, and also by the fact that the pore volume of electrode stack is filled 50-90% with electrolyte.
2. The cell described later by Dunlop et al. is characterized by hydrophobic negative electrodes, which can be biporous, and also by protection against polarity reversal by provision of a hydrogen precharge, as well as a volume of electrolyte which is not sufficient to flood the electrodes in the electrode stack.
All previously published practical embodiments of nickel oxide-hydrogen cells, including those of NASA-COMSAT, Tyco Labs., Eagle-Picher, ERC (Energy Research Corporation) and SAFT (Societe des Accumulateurs Fixes et de Traction) correspond to this embodiment.
The disadvantage of the cell, numbered 1 above, is the high operating pressure, which makes necessary a corresponding pressure housing and results in a decrease in energy density per unit weight. Another disadvantage of the high pressure, which is obviously necessary to allow the negative electrodes to function, is a higher rate of self-discharge of the cells, since this is proportional to the hydrogen pressure.
On the other hand, the cell described under No. 2 above has the advantage that the reaction zone in the negative electrode, the zone in which hydrogen gas, electrolyte, and catalyst-coated electrode structure come together, is defined by hydrophobization. The negative electrode therefore operates satisfactorily even at lower hydrogen pressures (with correspondingly lower solubility of the hydrogen in the electrolyte). Cells manufactured heretofore according to this design were designed for a maximum operating pressure of 34-40 bars.
In addition to the advantages described, such negative electrodes when used in nickel oxide-hydrogen cells suffer from certain disadvantages: It is generally known that hydrophobized electrodes "age", i.e. the degree of power capability of the electrodes decreases with operating time. The reaction zone, the location of the electrochemical reaction, is displaced with increasing age of the cell, and results in a decrease in the power rating. Another disadvantage, which appears especially in the nickel oxide-hydrogen cells of the type described under No. 2 above, is that the separators "dry out" with increasing operating time. This can be attributed for example to the fact that the electrolyte, which emerges from the electrode units through the hydrophobized back side, i.e. the side of the hydrogen electrode which delimits the gas chamber -- this phenomenon is called the "weeping" of the electrodes -- is irreversibly lost, since the hydrophobization prevents such electrolyte from returning into the electrode stack.
Another influence that contributes to limiting the lifetime of previously known nickel oxide-hydrogen cells is the swelling of the positive electrodes. Thus, an increase in the thickness of positive electrodes amounting to 40-70% in the course of 1,000 cycles has been reported, leading to a reduction of the utilization of active mass in the nickel oxide electrode and an increase in the ohmic resistance. This swelling, in fact, can be eliminated by a volume of active mass which is reduced relative to the volume of the positive electrode, but it is obvious that this measure also undesirably reduces the energy density of the cell as a whole.